Iran's nuclear chief said Wednesday that Tehran has invited the head of the International Atomic Energy Agency (IAEA) Rafael Grossi to visit in May.

The UN agency, based in Vienna, has been struggling since 2021 to carry out controls on Iran's nuclear programme, which continues to expand even as Tehran denies it wants to make nuclear weapons.

Last month, Grossi expressed frustration over Iran's nuclear activity in an interview with AFP, saying Tehran was "restricting cooperation in an unprecedented way".

The head of the IAEA "has expressed his desire to go to Iran and this trip is on his agenda," Mohammad Eslami, head of Atomic Energy Organisation of Iran, told reporters.

"We have invited him to give a speech at the international conference on nuclear energy" which will be held from May 6 to 8 in the central city of Isfahan, he added.

Iran suspended its compliance with limits on its nuclear activities set by a 2015 nuclear deal with major powers a year after then US president Donald Trump unilaterally pulled out of the agreement in 2018 and reimposed sweeping sanctions.

Iran last year slowed down its pace of uranium enrichment, which was seen as a goodwill gesture while informal talks began with the United States. But the IAEA said it accelerated enrichment once again in late 2023.

Ahead of his potential trip, Grossi met Wednesday in Vienna with the top US official on Iran, Abram Paley.

The US envoy told Grossi that "we remain seriously concerned about Iran's continued expansion of its nuclear program in ways that have no credible civilian purpose, including its continued production of highly enriched uranium,” State Department spokesman Matthew Miller told reporters in Washington.

Tehran has consistently denied any ambition to develop a nuclear weapons capability, insisting that its activities are entirely peaceful.

Earlier in February, Iran said it had started building a new nuclear research reactor in Isfahan, days after it announced it was constructing a nuclear power plant complex in the south.